It is the last line of defense on the football field, and serves to protect a player's most critical area: the head.

Helmet designs continue to improve as science discovers new ways to keep youth and adult athletes safe. However, complete concussion protection remains out of reach.

Now, one research team says their five star rating system can help families choose which helmet offers the lowest risk of concussion. During an investigation by WBIR's parent company, Gannett, 10News learned more about the study, which local football programs are using the results, and why some researchers caution against it.

The team

In 2013, Dr. Stefan Duma led a group – the Center for Injury Biomechanics -- a partnership between Virginia Tech and Wake Forest College. The team's research background spans from sports to auto safety and military.

About a decade ago, they decided to focus on concussion prevention.

"What separates football is you have a lot more head impacts, so the risk is the highest," Duma said.

The study

Simulating the impacts on a football field, Duma and his team measured head acceleration in a variety of different impacts. The measured how many times the simulated head took a hit, how hard, and how well it absorbed the blow.

After testing several different helmet models, the VT team built a list from their research. Then, they applied a "start rating" based on the results.

The lower the risk measured in their tests, the higher the star they gave. One brand earned a "No Recommendation."

"The differences in acceleration are dramatic between the one star and the very good four and five stars," Duma said.

"Sixty years of bio-mechanics all coalesces in this fundamental principle: lower acceleration, lower risk. It's true for head injuries."

The widespread impact

Three years after Duma's team first published their results, the study is gaining widespread attention. Football programs ranging from NFL to youth leagues are considering the star ratings as they debate new helmet purchases.

That group includes Anderson County High School Athletic Director Gary Terry.

"It's made us all more aware of the helmets we're putting on our kids' heads," he said.

Terry first learned of the ratings about two years ago. ACHS was already wearing mostly high-rated helmets, and he decided all new purchases would continue that trend.

ACHS buys about 10 news helmets each year, and Terry and head football coach Davey Gillum prefer the 4 and 5 star Riddell models.

"I'm pretty savvy about chasing down the best equipment and looking at price-wise and seeing where we fall and what we can afford and what we need to get right there," Terry said. "But ultimately – it's all about player safety -- and that's what we're trying to do here."

Helmets used in East Tennessee school districts(Photo: WBIR)

Local trends

10News requested helmet records from dozens of East Tennessee high schools. The results showed almost every school using helmets ranked three stars or higher.

Only two schools kept some helmets in their inventory that fall under the "not recommended" rating by Virginia Tech:

Halls and Gibbs High Schools both had the "Adams Pro Elite" helmet in their inventory – but not for much longer. Head coaches from those schools said the helmets are either not in use at all, or already being phased out for different models.

Both Halls and Gibbs primarily use Riddell and Schutt football helmets, which earn anywhere between a three and five star rating.

Despite the widespread attention, Virginia Tech's ranking system is just one study in an entire field of research. Some say it has serious limitations.

Dr. Irfan Asif specializes in sports medicine at UT Medical Center, and sees patients for a variety of head injuries, including concussions.

Asif points out, the study measures one type of acceleration – but that measurement doesn't provide a full picture of what causes a concussion.

"They're sort of saying -- with the linear accelerations, this is what happens -- but concussions can happen with rotational forces," Asif said. "So it's important to know that model does not take into account those rotational forces."

Perhaps the most vocal critic of the Virginia Tech study is an organization known for their role in the helmet industry. The National Operating Committee for Standards of Athletic Equipment (NOCSAE) certifies all athletic helmets for use in sports.

Dave Halstead is a research director at Southern Impact Research Center in Knoxville, and a consultant to NOCSAE.

"The merits of the science are actually reasonable. The merits of the conclusion are, in my opinion, unreasonable."

Like Irfan, he argues the data VT measured isn't applicable to concussions.

"Where I find that there's a problem, is where they say -- this helmet is a five star rating and will result in fewer concussions than this helmet is a four star -- there is zero science to support that. And it is a leap of faith that I think is premature," he said.

Both men caution coaches and parents not to rely on the ratings system as a guide to their helmet purchases.

"I would probably look for a helmet that meets current requirement and also fits well," Irfan said. "I think that's the important piece -- is find a helmet that fits well. You could have a five star helmet that doesn't fit you and it's not going to protect you."

Web extras

Watch Dr. Stefan Duma explain his research in the web extra below. You can also watch Dave Halstead argue against the ratings system in his full interview.